linux-kernel - Re: [PATCH v2 2/2] mm/page_alloc: use a single function to free page

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Message-ID: <d6b4890c-0def-6114-2dcf-3ed120dea82c@suse.cz>
Date:   Tue, 6 Nov 2018 09:16:20 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Aaron Lu <aaron.lu@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Paweł Staszewski <pstaszewski@...are.pl>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Yoel Caspersen <yoel@...knet.dk>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: [PATCH v2 2/2] mm/page_alloc: use a single function to free page

On 11/6/18 6:30 AM, Aaron Lu wrote:
> We have multiple places of freeing a page, most of them doing similar
> things and a common function can be used to reduce code duplicate.
> 
> It also avoids bug fixed in one function but left in another.
> 
> Signed-off-by: Aaron Lu <aaron.lu@...el.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

I assume there's no arch that would run page_ref_sub_and_test(1) slower
than put_page_testzero(), for the critical __free_pages() case?

> ---
> v2: move comments close to code as suggested by Dave.
> 
>  mm/page_alloc.c | 36 ++++++++++++++++--------------------
>  1 file changed, 16 insertions(+), 20 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 91a9a6af41a2..4faf6b7bf225 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4425,9 +4425,17 @@ unsigned long get_zeroed_page(gfp_t gfp_mask)
>  }
>  EXPORT_SYMBOL(get_zeroed_page);
>  
> -void __free_pages(struct page *page, unsigned int order)
> +static inline void free_the_page(struct page *page, unsigned int order, int nr)
>  {
> -	if (put_page_testzero(page)) {
> +	VM_BUG_ON_PAGE(page_ref_count(page) == 0, page);
> +
> +	/*
> +	 * Free a page by reducing its ref count by @nr.
> +	 * If its refcount reaches 0, then according to its order:
> +	 * order0: send to PCP;
> +	 * high order: directly send to Buddy.
> +	 */
> +	if (page_ref_sub_and_test(page, nr)) {
>  		if (order == 0)
>  			free_unref_page(page);
>  		else
> @@ -4435,6 +4443,10 @@ void __free_pages(struct page *page, unsigned int order)
>  	}
>  }
>  
> +void __free_pages(struct page *page, unsigned int order)
> +{
> +	free_the_page(page, order, 1);
> +}
>  EXPORT_SYMBOL(__free_pages);
>  
>  void free_pages(unsigned long addr, unsigned int order)
> @@ -4481,16 +4493,7 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
>  
>  void __page_frag_cache_drain(struct page *page, unsigned int count)
>  {
> -	VM_BUG_ON_PAGE(page_ref_count(page) == 0, page);
> -
> -	if (page_ref_sub_and_test(page, count)) {
> -		unsigned int order = compound_order(page);
> -
> -		if (order == 0)
> -			free_unref_page(page);
> -		else
> -			__free_pages_ok(page, order);
> -	}
> +	free_the_page(page, compound_order(page), count);
>  }
>  EXPORT_SYMBOL(__page_frag_cache_drain);
>  
> @@ -4555,14 +4558,7 @@ void page_frag_free(void *addr)
>  {
>  	struct page *page = virt_to_head_page(addr);
>  
> -	if (unlikely(put_page_testzero(page))) {
> -		unsigned int order = compound_order(page);
> -
> -		if (order == 0)
> -			free_unref_page(page);
> -		else
> -			__free_pages_ok(page, order);
> -	}
> +	free_the_page(page, compound_order(page), 1);
>  }
>  EXPORT_SYMBOL(page_frag_free);
>  
>